IV. Action of somatostatin on colonic smooth muscle cells. Recent studies demonstrate somatostatin (SS) has marked effects on colonic motility. To explore the cellular basis of action for the ability of SS to alter colon contractility, the effect of SS on dispersed muscle cells from guinea pig colon was investigated. The results of SS's action on colonic muscle cells was compared to those on gastric muscle cells, which have been well studied previously. Our results demonstrated that the action of SS differs in colonic and gastric smooth muscle cells. SS inhibits contractants in colonic cells and inhibits relaxants in gastric cells. In colonic cells from different muscle layers, SS has a weak contractile effect due to an effect on circular muscle cells and an inhibitory effect on cells from both longitudinal and circular layers. A different SS receptor subtype mediated the actions of SS in colonic and gastric muscle cells. In both cell types, the actions of SS were mediated by pertussis toxin-sensitive and -insensitive G proteins.